Apparatus for generating stable frequencies



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APPARATUS FOR GENERATING STABLE FREQUENCIES Filed Aug.- 4, 1955 7 Sheets-Sheet 5 y' LMABKQQGLLAAIO F/GZ June 3, 1958 H. J. KEEN ETAL 2,837,650

APPARATUS Foa GENERATINc STABLE FREQUENCIES A Filed Aug. 4. 1955 7 Sheets-Sheet 6 June 3, 1958 H. J. KEEN ETAL APPARATUS FoR GENERATING STABLE: FREQUENCIES 'r1 sheets-sheet 7 Filed Aug. 4. 1955y ,.lllllllllllllllmN l I .||I

United States Patent O APPARATUS FOR GENERATING STABLE FREQUENCIES Harry J. Keen, Atlantic Highlands, Peter Muller, Long Branch, Donald Colbert, Old Bridge, and Robert J. Papaieck, Sayreville, N. J., assignors to Lavoie Laboratories, Inc., Morganville, N. J., a corporation of New Jersey Application August 4, 1955, Serial No. 526,372 16 Claims. (Cl. Z50- 36) The present invention relates to apparatus for generating a plurality of stable frequencies of predetermined value. Apparatus in accordance with the invention is suitable for use in radio transmitting and receiving equipment where it is desired to be able to transmit and receive on different frequency channels. The invention is also applicable to test equipment and other apparatus where a stable source of frequencies of predetermined values is desired. While the invention is in no way limited to any frequency range, it is particularly useful in the generation of ultrahigh frequencies where a large number of individual frequencies extending over a wide range are to be generated. By way of example, and without limitation, the invention is herein described with reference to apparatus for generating frequencies at kilocycle intervals in the range of 200 to 400 megacycles.

Frequency-generating apparatus heretofore available has generally employed one or more crystal oscillators or other frequency sources the output of which is passed through a rather complex system of multipliers, dividers, mixers, etc. to obtain the various frequency channels desired. When it is desired to obtain a large number of closely spaced frequency channels over a wide range, such systems become rather complex and somewhat erratic in their operation. The present invention differs basically from these prior systems in that the output frequency is obtained directly from a variable frequency oscillator which is variable over the entire selected range and the frequency of which is automatically controlled with extreme accuracy. Among the features and advantages of apparatus in accordance with the invention are its simplicity and its adaptability to small size and light weight. Not only does the apparatus require fewer components but its performance characteristics are suitable for the use of transistors, thereby further facilitating miniaturization. The apparatus is remarkably free from spurious responses and is exceptionaly stable. A stability of 0.0025 is obtained with commercial components and even higher stability is possible if required. The frequency selection can be effected by remote control, for example through a dial or push-button system and no manual adjustments are required within the system to change from one channel to another. When a new frequency is selected, the apparatus quickly and automaticaly tunes to the selected frequency. Two or more oscillators can be controlled through a single tuning system without the complication of a side-step arrangement.

Other features, characteristics and advantages of the invention will become apparent from the following description and claims and from the accompanying drawings which illustrate a preferred embodiment and in which:

Fig. 1 is a simplified block diagram of apparatus in accordance with the invention shown in conjunction with a receiver and a transmitter;

Figs. 2A to 2F are, together, a schematic wiring dia- ICC gram of apparatus in accordance with the invention, in association with a transmitter;

Fig. 2G is a diagram showing the arrangement of the sheets comprising Figs. 2A to 2F; and

Fig. 3 is a chart illustrating the operation of the apparatus.

The general principles of operation of frequency-generating apparatus in accordance with the invention will be understood from the following description with reference to Figs. l and 3. In Fig. l, the frequency-generating apparatus is shown associated with a transmitter and a receiver in such a way that the same control circuits can, by a simple switching operation, be used to control the frequency of the transmitter or receiver alternatively. The transmitter is shown as comprising a master oscillator O1, a-buffer amplifier, a driver amplifier and a nal amplifier connected to the antenna. The master oscillator O1 is also connected to a master mixer X1. The oscillator, mixer and amplifiers are ganged together and are tunable by means of a course-tuning motor M1 and a fine-tuning motor M2, the term motor being herein used generically to include an electric or hydraulic motor or other suitable power unit.

The receiver is shown as comprising an antenna-tuning circuit, a radio frequency amplifier, a master oscillator O1 and a receiver mixer. The master oscillator O1' is also connected to a master mixer X1. The antennatuning circuit, radio frequency amplifier, receiver mixer, master oscillator and master mixer are ganged together and tuned by means of a course-tuning motor M1' and a fine-tuning motor M2'. The apparatus is switched from Transmit to Receive by means of a relay S1 having operating contacts Sla, S1b, Slc and Sld with the connections shown (Fig. l). The master oscillator, master mixer and tuning motors of the receiver correspond to like units of the transmitter and are controlled and operate in like manner. Where reference is hereinafter made to the master oscillator, it may be eitherl the oscillator O1 of the transmitter or the corresponding oscillator O1 of the receiver, depending on the operative position of the relay S1. The same is true of the master mixer and the tuning motors.

The master oscillator is tunable over the entire range of frequencies to be produced, for example from 200 to 400 megacycles. Initial tuning of the master oscillator to approximately the selected frequency is effected by means of the coarse-tuning motor M1 under control of a coarse-tuning circuit. The oscillator is then brought to the selected frequency by means of the fine-tuning motor M2 under control of a fine-tuning circuit comprising, in addition to the master oscillator and master mixer, a marker oscillator O2, a wide-band amplifier, a harmonic generator, a sweep oscillator, a motor direction control circuit, a disabling circuit and an anti-hunt circuit. The entire system is controlled by means of a control box or panel which may, if desired, be located a considerable distance from the rest of the equipment so as to provide remote control. As will be described in more detail below, the control box may include a dial, pushbutton or selector switch arrangement for setting up the desired frequency.

The marker oscillator provides a series of rather widely spaced harmonic frequencies which serve as markers or reference points for tuning the master oscillator. In the illustrated embodiment, the marker oscillator provides a series of frequencies spaced four megacycles apart over the entire selected range of the equipment. A portion of the range is illustrated in Fig. 3 where marker frequencies are indicated at 232, 236 and 240 megacycles. The coarse-tuning unit is sufficiently accurate to bring the master oscillator to within two megacycles of the marker frequency nearest the selected frequency and on the broad-band amplien 3 the :correct side of--above or belowtlie marker frequency. For examplef the frequency selected is 237.3 megacycles, the nearest marker frequency is 236 and the coarse-tuning unit is sufciently accurate to tune the master oscillator tok a frequency between 236 and 238 megacycles. If, on the other hand, the selected frequency is below thenearest markerfrequency, for example 234.4 megacycles, the coarse tuning willbriug the master oscillator to va frequency somewhere between 234 and 236 megacycles. No tuning of the marker oscillator is required since the rparticular marker frequency that is to be used as a reference in tuning the master oscillator to a selected frequency is determined by the coarse tuning. The marker oscillator needs to be accurate and stable in order to provide accurate and stable reference points for the tuning of the master oscillator.

V'After the coarse tuning of the master oscillator has been completed, the .oscillator is brought accurately to the selected'frequency by the tine-tuning motor M2-under control of the line-tuningv system. As the coarse tuning may result in a frequency which is either above or below the selected'frequency, the fine tuning circuit includes a control circuit for automatically determining the proper direction of rotation of the fine-tuning motor. For example, if the selected frequency is 237.3 megacycles, the coarse tuning'may leave the master oscallator at a point A (Fig. 3) below the selected frequency Yor at a point B above 'the selected frequency. VIn either event, the motor direction control circuit--in cooperation with the marker oscillator, master mixer, broad-band amplifier, sweep oscillator and disabling circuit-determines the direction of the fine-tuning motor-reversing the motor if necessary-so that it will dn've .the master oscillator in a direction toward the selected frequency. YWhen the selected frequency lis reached, the fine-tuning motor is stopped by means ofan anti-hunt circuit in cooperation with the marker oscillator, master mixer, broad-band amplifier and harmonic generator.

The harmonic generator provides a series of frequencies at intervals equal to the desired intervals 'at which the master oscillator is to be tuned and extending over a range equal to at least half the'interval between successive marker frequencies. ln the illustrated embodiment, the harmonic generator'provides frequencies over a range of 2 megacycles in 100 kilocycle steps. If closer channel spacing is desired, the frequency steps of the harmonic generator should be correspondingly reduced. The harmonic generator includes a frequency selector or filter which is controlled from the control box to'select a particular harmonic frequency that, when added to or subtracted from a marker frequency, will result in the frequency to be selected. For example, if the frequencyselecting means of the control box is set for a frequency of 237.3 megacycles, a frequency of `1300 kilocycles is provided by the harmonic generator. This, addedV to the nearest marker frequency of 236 megacycles, equals 237 .3 megacycles. It will be seen that the inputs of the master mixer X1 are connected with the master oscillator band with the marker oscillator. The anti-hunt circuit is connected with the harmonic generator and also with the output of the master mixer amplified by When the master oscillator driven by the finer-tuning motor reaches a frequency that equals the algebraic sum of the nearest marker frequency and the frequency provided by the harmonic generator, the motor is turned off, thereby stopping the oscillator at the selected'frequency.

The fine-tuning circuit also .includes la zero-beat tuner the purpose of which Yis to provide for tuning the master oscillator at one or another of the marker frequencies. "This circuit, together with the other circuits referredto n a general way above, will no w be described in more etail.

drawings, reference will bemadeonly VtoV such elements Howevenas the circuitry is clearlyV shown in the4 and'connections as'are necessary'to aclear understanding of the invention.

Control box The control box (Figs. land 2F) comprises selectors operated either automatically or manually for setting up the desired frequency to which the master oscillator is to be tuned. As illustrated in Fig. 2F, the selectors comprise four banks of switches or contacts, one bank for each digit of the frequency to be selected. Thus, bank D represents the irst digit, bank Ethe second, bank F the third and bank G the fourth. Bank D comprises a single series of contacts D1 numbered 2 and 3 respectively. Bank E comprises two series of contacts El and E2, each having ten contacts numbered 0 to 9. Bank F comprises three series of ten contacts each, the series being Vdesignated F1, F2 and F3, respectively. Bank G comprises a single series G1 of ten contacts. The contacts are operated in any suitable manner, as by push-button or dial system. For example, the lcontacts represented in the drawings may be considered as being contacts of stepping relays engageable by movable arms or runners which are represented in the drawing by arrowheads. The runners of contact series E1 and E2 are ganged together as are also those of contact series F1, F2 and F3. The number and arrangement of the contacts or selectors depends on the frequency range to be covered and the spacing of the steps or channels. The arrangement illustrated in Fig. 2F is intended to cover a frequency range of from 200 to 400 megacycles by steps of 100 kilocycles. The runners are shown in position to select a frequency of 237.3 megacycles.

Contact series D1, El Vand F1 together control the coarse positioning of the master oscillator and, for this purpose, Iare connected by a lead 11 to the coarse tuning circuit (Fig. 2C). The coarse positioning brings the master oscillator to a position within Vtwo megacycles of the marker frequency nearest the frequency to be selected. Contact series E2 and F2 deter-mines whether the selected frequency is above or below the nearest marker frequency and, for this purpose, are connected by a lead 12 to a reversing relay YS2 in the fine tuning circuit (Fig. 2C). Contact series F3 and G1 control a stepping relay S3 (Fig. 2C) for selecting a frequency from the harmonicV generator to be added to, or subtracted from, the nearest marker frequency and, for this purpose, are connected by frequency selected by the stepping relay is added to, oi"

is subtracted from, the marker frequency. The contacts of the several series are interconnected by the circuits clearly shown in Fig. 2F. Standard resistances R1, R2, R3, R4 and R5 are connected between certain of 'the contacts, as shown, nso that'predetermined resistance values can be set up by the selectiveV operation of the contacts. A switch S4 for initiating the tuning operation is also preferably located in the control box although, for convenience, it is shown in the coarse tuning circuit (Fig. 2C).

Coarse tuning The coarse tuning of the master oscillator O1 is effected by the Vcoarse'tuning motor M1. For this purpose, the motor M1 is connected by'mechanical or other connecting meansl with a-Variable capacitor C1 or other tuning element ofthe master oscillator and also with other units Y. to be tuned, for example the buffer amplier, nal amplier and master mixer@Y The coarse tuningmotor M1 is also coupled with an'auxiliary motor MlA which, ina tuning cycle, initially acts as a motorv assisting the motor M1 in quickly effecting the coarse tuning and thenacts as a generator to prevent huntingj as will be explained. The motors M1 and MIlA are permanent'rnagnet motors and hence are reversible by reversing the direction of the armature current. The motors are controlled by a coarse tuning circuit (Fig. 2C) which comprises essentially a l). C. amplifier and a resistance bridge. Three legs of the bridge comprise resistances R6 (Fig. 2F), R7 and R8 (Fig. 2C). The fourth leg of the bridge vcomprises a variable resistance R9 which is operable by the motor M1 and is connected in series, by lead 11, with whichever resistances R1, R2 and R3 have been set up by the series of contacts D1, E1 and F1 in the control box. The D. C. amplifier comprises two triodes V1A and V1B having their plates connected respectively to the two operating coils of a double-throw relay S5. The grid of the tube VIA is connected to the resistance bridge at a point between the resistances R8 and R9 while the grid of tube VlB is connected to ground and hence in elect is connected to the resistance bridge between resistances R6 and R7. Terminals 14a and 14h are connected respectively to the opposite sides of a battery or other power source that is not connected to ground and is thus termed lioating with respect to ground. Initial balancing of the tubes VlA and VlB is provided by the circuitry shown, including a zero-balancing relay S6. When the relay is de-energized, contacts S611 connect the grid of tube VIA to ground and contacts 56a` connect the central contact of relay S5 to the grid of tube V1B through resistances R16 and R11. A condenser C2 is connected between the grid of tube VIA and ground while a condenser ()3 is connected between ground and a point between resistances R10 and R11. The two opposite contacts of relay S5 are connected respectively to positive and negative supply. Hence, if the movable central contact of relay S5' engages either of the side contacts due to any imbalance in the plate currents of tubes V1A and VlB, a positive or negative voltage is applied to the condenser C3 and, through the resistance R11, to the grid of tube V13 to correct the unbalance.

At the beginning of a tuning cycle, the zero-balancing relay S6 is energized by current supplies through a lead 16 from a relay in the tine tuning circuit that will be described below. Relay S6 will thereupon be kept energizedthrough holding contacts Sc. When the relay S6 is energized, its contacts S760 connect the central contact ot` relay S5 with the coil of a polarized slave relay S7 so that the latter relay follows S5. A motor control relay S8 is momentarily energized at the beginning of a tuning cycle by current supplied through the contacts of switch S4 which, as described above, is in the control box and is actuated to start the cycle. The relay S3 will there upon be kept energized through holding contacts 88a'. The coarse tuning motor M1 is connected to the central contact of the slave relay S7 through switch S4 which will have been released and returned to the initial position as shown. The armature of the motor M1 will thereby be connected to either a positive or negative source, depending on the position of a slave relay S7 which in turn depends on the position of the relay SS controlled by the plate current from tubes V1A and V1B. The motor M1 is hence caused to rotate in the proper direction to vary the resistance R9-connected to the motor-so that the combined resistance of R9 and the resistance that has been set upby contacts of series D1, E1 and F1 in the control box `balance the resistance RS. The auxiliary motor M1A is connected in parallel with the motor M1 through contacts Stic and hence helps the coarse tuning motor. When the resistance bridge passes through its balance pointi. e. when variable resistance R9 together with selected resistancesfll, R2 and. R3` balance resistance RS-the relay Si? will "reverse, thereby reversing the polarity of current supplies to the'motor M1 and relay S8 andauxhiary motor M1A will be de-energized, but the system comprising the. motors'Ml and M1A and connected units will go beyond the zero pointbecause of its inertia;v The de-euergizing of the relay S8 causes contac'ts S51: connect lthe auxiliary lmotor M1A through a lead 17 to the runner of an adjustable resistance R12 connected betweenfthe grid of the tubeVlB and ground. As Vtheinotor Ml'reverses and startsturning in the op- CTI posite direction to bring the system back to the balance point, motor M1A acts as a generator and the current that it generates produces a voltage in the resistance R12 opposing the voltage resulting from the unbalance-due to overshooting--of the resistance bridge. The counteracting eltect of the motor M1A causes the system to come back to the balance point by a series of decreasing steps so that balance is effected quickly but without hunting. When equilibrium is established, the resistance of variable resistance R9, added to such of resistances R1, R2 and R3 as have been set up in the control box, balances resistance R8. The connections between the coarse tuning motors, the Variable resistance R9 and the master oscillator O1 are such that, when equilibrium is established, the master oscillator is at, or near, the selected frequency that has been set up in the control box within the permissible tolerances of the coarse tuning, as indicated above. The same is true of the other tunable units associated with the master oscillator, such as the buier amplifier, linal amplilier and master mixer X1. The motor control relay S8 also has contacts 88a connected in series with holding contacts S6c of relay S6 and contacts SSI) which are connected by leads 18 and 19 with a relay in the fine tuning circuit that will be described. When the coarse tuning is completed, the `coarse tuning system is mechanically locked.

Fine tuning The line tuning of the master oscillator O1 is effected by the tine tuning motor M2 which, for this purpose, is connected by mechanical or other connecting means 20 (Figs. 2C and 2F) with a tunable element 21 of the master oscillator, illustrated as a tuning slug. The motor is also connected to suitable tuning elements 21a, 2lb and 21e of the buffer amplifier, final amplifier and master mixer X1, respectively. An auxiliary fine tuning motor MZA is shown coupled with the motor M2 so that the two run together. The tine tuning motors are in part controlled by a homing relay S10 and by microswitches S11 and S12 actuated by a cam 22 coupled to, and driven by, the line tuning motors. The homing relay S10 and cam-actuated switches S11 and S12 are designed to bring the tunable element 21 of the master oscillator to a selected point in its tuning range. The point selected is preferably about midway of the range so that tuning time is minimized. When the relay S8 of the course tuning circuit is energized, as described above, contacts S81) closed. Assuming that the tine tuning motor is already not in home" position, one or the other of cam-operated switches S11 and S12 will be closed so that the relay S10 is energized by current supplied through lead 19, contacts SSb, lead 18 and one or the other of cam-operated switches S11 and S12. The relay is then kept energized by holding contacts S until the line tuning motors reach home position, whereupon both of the cam-operated switches S11 and S12 are opened and relay S10 is deenergized. While lthe homing relay S10 is energized, current is supplied to motor M2 through contacts S10b and to motor M2A through contacts 810e. The two motors M2 and M2A thus work together' in bringing the fine tuning system comprising the motors and the several tuning elements to home position. When the homing relay is thereupon cle-energized, the contacts S1011 short-circuit the armature of the motor MZA so that it operates as a generator and acts as a dynamic brake on the motor M2 to damp the operation of the latter motor in the tuning operation described below and thereby assist in preventing hunting. The motor M2 is connected through contacts S10b and leads 23 and 24 with the contacts of an anti-hunt relay S13 (Fig. 2E) and a direction control relay S14 (Fig. 2C). The relay S14, together with the contacts S2b and S2c of relay S2 control the direction of the motor M2 by connecting it| alternatively with a-negative or positive power supply. The motor is thereby caused to run in the proper direction to tune the master oscillator and associated units to the selected frequency. When this frequency is reached, the anti-hunt relay S13 Vopens to stop the motor. The anti-hunt relay S13 also controls a power supply relay S (Fig. 2C) having contacts S15a and S15b for supplying power to certain of the circuits hereinafter described through leads and 26, respectively.V When the tune-up is completed, the opening of the anti-hunt relay S14 deenergizes relay S15 and thereby cuts off the power supply of the circuits connected toi the leads 25 and 26 to Vavoid unnecessary energization of these circuitsV and thereby lengthen the life of theV equipment. The relay S15 also has contacts 815e through which the zero-balancing relay S6 of the coarse tuningvcircuit is energized when these contacts are closed. v The circuits controlling the motor direction relay and anti-hunt relay will now be described. The outputs of the master oscillator O1 and the marker oscillator O2 are connected by leads 27 and 28, respectively, to the inputs of the master mixer X1, for example, a cavity type mixer as shown. The output of the master mixer X1 is connected by a lead 29 to the wide-band amplifier A1 (Fig. 2D). By mixing the output signals of the master oscillator and marker oscillator, the master mixer X1 produces a beat having a frequency that is equal to the difference of the frequencies of the two oscillators. The beat signal, amplified by the wide-band amplifier A1, is compared by the circuits described below with a frequency selected by the setting of the control box from the frequencies produced by the harmonic generator. If the beat frequency is lower or higher than the harmonic generator frequency, the ne tuning motor is energized to run in the proper direction to vary the frequency of the `master oscillator rvsoas to equalize the frequencies being compared. When the frequency of the beat signal `ofthe master oscillator equals the selected frequency of Vthe Vharmonic generator, the anti-hunt relay S13 is opened to stop the tine tuning motor.

The masteroscillator O1 (Fig. 2F) comprises a tube V2 having a heated cathode, control grid and plate connected inan oscillating circuit, as shown in the drawings'. The frequency of the master oscillator O1 is variable over the entire range that is to be supplied, the frequency being variable by means of the variable capacity C1 and the movable slug 21 for varying the inductance of a coil L1, thecapacitance C1 being varied by the coarse tuning motor-,and the vslug 21 being movable by the tine tuningv motor, as described above. The marker oscillator O2 (Fig. 2D) is a crystal orrother oscillator capable of producing harmonics of xed frequencies which Vare used as reference points or markers in tuning the master oscillator. For example, the marker oscillator O2 provides harmonic frequencies at intervals of 4 megacycles. The oscillator is illustrated as comprising a crystal 30, tubes VSA, VSB, V4 and V5 connected in an oscillating circuit, as clearly shown in Fig. 2D. The master mixer X1 (Fig. 2F) ,is preferably a cavity-type mixer having a variable capacity C4 controlled through connections 15 by the coarse tuning motor M1 and an inductance L2 variable kby a slug 21e controlled through connections 20 bygthe fine tuning motor M2. The ini puts of the master. mixer X1 are connected by the leads 27 and 28 to the master oscillator O1 and marker oscillatorOZ, respectively, while the output is connected by the lead 29 to the wide-band amplifier A1. The wideband amplifier (Fig. 2D) comprises tubes V6, V7, V8 and V9, 'together `with the other elements andconnec-A tions shown. i

The harmonics generator (Fig. 2E) comprises a crystal 30, dual triode V10, tube V11 and blocking oscillator transformer'Tl, together with the other elements and connections illustrated in the drawings. In the present example, theharmonics generator provides a series of harmonic frequencies spaced 100 kc. apart. Harmonics of a' closer orlwider Vspacing maybe used, depending on the desired spacing of the frequency channels of the master oscillator. Instead of using a separate oscillator, it is possible' to'derive the 100 kc. or other harmonics from the marker" oscillator, for example by two divider stages. This would result in'a single crystal system. However, it has been found more convenient to use an additional crystal oscillator for producing the 100 kc. harmonics instead of dividers. The output of the harmonics generator is fed to a harmonic selector which selects a particular harmonic, depending on the setting of contacts of the series E2, F2, F3 and G in the control box (Fig. 2F). The harmonics selector (Fig. 2E) comprises the contacts ot levels S3b, S30, S3d and S3e of the stepping relay S3 (Fig. 2C).V are connected to condensers of predetermined value while selected contacts of levels 83d and S36 are connected to inductances. The capacitances and inductances are connected so as toprovide a resonant circuit the resonant frequency of which is determined by the position of the moving contacts of the stepping relay S3. The relay S3 is stepped in a clockwise direction by means of a stepping actuator or motor '53m under control of a circuit comprising a double-throw relay S16, a dual triode V12, resistances connected to the control contacts S3a of the stepping relay, the reversing relay S2 and contact series E2, F2, and G1 of the control box including the resistances R4 and R5. When a frequency value is set up in the control box, the relay S3 is stepped by the motor 83m until equilibrium is established between the resistance determined by the setting of the control box contacts and the resistances connected to the control contacts 83a of the stepping relay. rl`he contacts S2a of the reversing relay S2 determine whether the resistance of the control box are connected to one end or the other of the series of resistances connected to the control contacts 83a.

Hence, the position at which the stepping relay stops is determined not only by the resistance value set up by contacts F3 and G1 of the control box but also on the position of the reversing relay as determined by the set'- ting of contacts of the series E2 and F2. This depends in turn on whether the selected harmonic from the harmonics generator is to be added to or subtracted from the marker frequency supplied by the marker oscillator. The contact arms of all levels of stepping relay S3 are connected together as indicated by the broken line 31. The output of the harmonics selector is connected through a cathode follower V13 to an output lead 32. i

The circuits controlling the direction of the ne tuning motor include the sweep oscillator O4 (Fig. 2A), the motor direction circuit and the disabling circuit (Fig. 2B). The sweep oscillator comprises a sawtooth current generating circuit containing a` gas-filled tube V15, resistance R14, capacitance C5, yvacuum tube'V16 and coil L4 and an oscillatory circuitV O5 containing a dual triode V17, inductance coil L5 and stray capacitance C6. Power is supplied through leads Z5 and 26 and contacts S15a and S1511 of relay S15 (Fig. 2C). When the relay S15 is energized, the lead 25 is connected to a negative voltage source. A negative voltage is thereupon applied to the grid of the tube V15 and the grid is initial- 1y biased-with respect to the cathode by reason of the voltage drop across the resistance R14. The plate is grounded through a resistance R15. As the condenser C5 becomes charged the flow of current through resistance R14 and hence the bias between the grid and the cathode decreases .andthe tube ignites, discharging the condenser C5. The tube V15 is extinguished when the condenser is discharged. The flow of current through the resistance R14 as the'condense'r C5 again starts' to change, re-establishes the bias between the grid and cathoder This cycle is repeated, for'fexample at a rate of approximately ve times 'ipersecond-.f'The control grid of tube V16 is also connected to thelead 2.5 through a resistance R17 and' to the control` grid of tube V16' through a resistancelplr. A At the Ybeginning of thecycle,

Selected contacts of levels Sb and S3c 'the plate current of tubev V16 owing through coil L4 is at a maximum. It gradually decreases as the condenser C becomes charged and the negative voltage on the control grid builds up because of the decreased voltage drop across a resistance R17 in the lead 25. The coils L4 and L5 are both on the same core which is formed of a ferramic material having the property that its inductance is decreased by the direct current in the coil L4. Hence, as the plate current of tube V16 flowing through coil L4 decreases, the inductance of the coil L5 in the oscillatory circuit OS increases, thereby progressively varying the frequency of oscillation of this circuit. The sweep oscillator thus has the characteristic of producing a sawtooth frequency curve which, in the present example, has a sweep from approximately 2500 kc. to 85 kc. recurring at a rate of approximately five cycles per second.

The disabling circuit (Fig. 2B) comprises a mixer X3 containing a tube V18, an amplifier A3 containing a tube V19 and including a band pass filter P2 and a gas-filled switching tube V20. The mixer X3 has two inputs one of which is connected by the lead 32 with the output of the harmonic selector (Fig. 2E) while the other is connected by a lead 33 with the output of the sweep oscillator O4. The output of the mixer is fed to the amplifier A3 and band pass filter P2, the output of which controls the switching tube V20. The filter has a band width of, for example, 6 kc. When the frequency of the sweep oscillator equals the selected frequency from the harmonic selector within the band width of the band pass filter P2, the switch tube V20 ignites and supplies current through a lead 34 to disable the motor direction circuit. When the tube V of the sweep oscillator O4 (Fig. 2A) fires, as described above, the condenser C5 provides a sudden rush of current which is fed through a lead 35 to the switch tube V and extinguishes the latter, thereby resetting it for the next cycle of operation.

The motor direction circuit (Fig. 2B) comprises a mixer X4 including a tube V21, an amplifier circuit A4 including a low pass filter P3 and a gas-filled switching tube V23. The mixer X4 has two inputs one of which is connected by lead 33 with the output of the sweep oscillator (Fig. 2A) while the other is connected by a lead 36 with the wide band amplifier A1 (Fig. 2D). The wide band amplifier thus feeds to the input of the mixer X4 the amplified output of the master mixer X1 to the inputs of which the master oscillator O1 and the marker oscillator O2 are connected as described above. The output of the mixer X4 is fed to the amplifier A4 and low pass filter P3, the output ofA which is connected to the control grid of the switching tube V23. When the frequency of the sweep oscillator equals the output frequency of the master mixer X1 within the band widt-h of the filter YP3, for example 6 kc., the switch tube V23 `will ignite, providing it has not already been disabled by thedisabling circuit. The control grid of the switch tube V23 is connected through the lead 34 and a resistance R18 to the plate of the switch tube V20 in the disabling circuit. The plate of tube V20 is normally at zero potential while the cathode is negative. When the tube V20 fires, as described above, a negative voltage is applied through the resistance R18 and lead 34 to theV control grid of the switch tube V23. If this occurs prior to the beat produced by mixer X4, the motor direction circuit is disabled in the sense that the switch tube V23 will not be fired by la pulse produced by a beaty in mixer X4. lf, on the other hand, the beat from mixer X4 occurs prior to the beat from mixer X3, the switch tube V23 ignites and supplies current through leads 37 and 38 to the operating coil of the motor direction control relay S14 (Fig. 2C). rl`hisrelay is spring-biased so that the movable Contact is normally in the position shown, being moved to the opposite position by the energizing of the operating coil. Hence, when the coil isv energized, the direction of the fine tuning motor is reversed by` reason of the connections shown in Fig. 2C

10 and referred to above. The plate circuit of the switch tube V23 automatically resets itself after a predetermined time interval which, in the present instance, is approximately one-fifth of a second, corresponding to the cycle of the sweep oscillator.

In Fig. 3, the frequency range of the sweep oscillator O4 is shown superimposed on a portion of the frequency scale of the master oscillator O1 and the direction of the sweep is indicated by an arrow. rhe selected harmonic frequencies produced by the harmonics generator and selected by the harmonic selector (Fig. 2E) are also indicated. Assuming that a frequency of 237.3 megacycles is to be selected and has been set up in the control box, the coarse tuning motor will position the master oscillator somewhere in the range between 236 and 238 megacycles and the stepping relay S3 will be stepped to a position to select a harmonics generator frequency of 1300 kc. which, added to a marker frequency of 236 megacycles, will equal 237.3 megacycles. Assuming that the coarse tuning motor positions the master oscillator at a point A, approximately 236.7 megacycles, the beat frequency of the master mixer X1 will be 236.7 minus 236, or 700 kc. As the frequency of the sweep oscillator sweeps down from approximately 2500 kc. to 85 kc., it will reach the selected frequency-1300 kc.- of the harmonics generator before the frequency-700 kc.-of the master mixer output. Hence, the switch tube V20 of the disabling circuit will be fired iirst and will disable the motor direction circuit so that thc tube V3 of the latter circuit will not fire. This causes the relay S14 to remain in the position to which it is biased. The connections of the fine tuning motor are such that, when the relay S14 is in this position, the master osciilator is tuned in a direction to increase its frequency and hence toward the selected frequency of 237.3 megacycles. If, on the other hand, the coarse tuning motor positions the master oscillator to the position at a frequency indicated in Fig. 3 by the point B approximately 237.7 megacycles, the beat frequency of the master mixer will be 1700 kc. and hence above the selected frequency-l300 1re-of the harmonics generator. Hence, the frequency of the sweep oscillator will coincide with that of the master mixer before it reaches the selected frequency of the harmonics generator. This results in the switch tube V23 of the motor direction circuit being fired before being disabled by the disabling circuit. The current resulting from the firing of the switch tube V23 energizes the motor direction control relay 8f4 and thereby changes the direction of the fine tuning motor so as to decrease the frequency of the master oscillator and thus tune the oscillator in a direction toward the selected frequency.

When the master oscillator reaches the frequency that has been selected, the fine tuning motor is stopped by the antihunt circuit shown in Fig. 2E. rfhis circuit comprises a mixer XS, wit-h a dual triode 25, a fil e" plier A5 containing a dual triode V26 and a V27 the plate of which is connected to thc op of the anti-hunt relay S13. The tubes V25 c i' have no connections to their plates other tht... through a capacitator C8. This has the characte of favoring first-order mixing. The mixer X5 has two inputs one of which is connected by the lead to the output of the harmonic selector while the other is connected by the lead 36 to the output of the wide band amplifier (i"; 2D) for the master mixer. The output of the mixer :i is fed to the filter amplifier A5 which comprises a low pass filter that is llat from 0 to l0 kc. and cuts off at l0 kc. The rectified output of the low pass filter is fed to the grid of the switch tube V27 which is a hard tube. When the output frequency of the master mixer X1, amplified by the wide band amplifier, equais the selectedfrequency of the harmonics'generator within the band width of l0 kc., the switch tubey V27 is cut l off, thereby deenergizing the relay S13 and stopping the quency from the harmonics generator is-in the example illustrated-1300 kc. When the master oscillator, as it is being tuned by the fine tuning motor, reaches a point at which the algebraic sum of its frequency and the nearest marker frequency-236 megacycles-equals 1300 kc., the anti-hunt circuit functions as described to stop the line tuning motor. Hence, the master oscillator stops at the selected frequency of 237.3 megacycles. Actually, in the present example, the tine tuning motor is turned off 10 kc. before the master oscillator reaches the selected frequency. As the ne tuning motor and its operative connections necessarily have some inertia despite the braking eect of the auxiliary motor M2', this inertia compensates for the l lic. band width of the anti-hunt circuit. The band width of the filter can be reduced or increased, as desired, depending on the required accuracy and the inertia of the tuning system.

Zero beat tuner Since the sweep oscillatordoes not go all the way to zero and since directional control is needed, a zero beat tuner is provided for tuning in those instances where the desiredfrequency coincides with one of the marker frequencies, for example 236 megacycles. The zero beat tuner comprises an amplifier containing a tube V28, a voltage doubler with tube V29 and a switch circuit including a tube V30 controlling a relay S20. The tube V30 is a sheet beam tube with a split anode.

The relay S20 is a double throw polarized relay with two operating coils connected respectively tothe anodes of tube V30 and having the characteristic of staying in whichever position it is left until the opposite coil is energized. The zero beat tuner is put into operation by means of a relay S21 the operating coil of which is connected by a lead 44 to the zero position contact of the contact series S36 of the stepping relay S3 (Fig. 2E). The'zero position contact of the contact series 83e of the stepping relay is connected by a lead 39 to a suitable power source while the runners of the two levels SSC and 53e are connected together by a lead 40. Hence the relay S21 is energized when the stepping relay S3 is in zero position which will be the case when the frequency selected is one of the marker frequencies. The relay S21 has three sets of contacts of which contact S21a connect one of the contacts of relay S20 through leads 40 and 38 to the operating coil of the motor Vdirection control relay Sie (Fig. 2c). Contacts SZlb connect the zero beat tuner through lead 4l to a suitable power source. Contacts SZic connect a lead 42 from the sweep oscillator circuit through a resistance to ground thereby disabling that circuit.

The input of the amplifier A5 is connected by leads 43 and 36 to the output of the Wide band amplifier A1 (Fig 2d). The output of the master mixerV X1 amplified by the wide band amplifier A1 is thus fed to the input of the amplifier A5 of the zero beat tuner. As the inputs of the master mixer X1 are connectedrespectively to the master oscillator O1 and the marker oscillator O2 and as the master oscillator will have been tuned by the coarse-tuning motor to a frequency near the selected marker frequency, a beat will occur with a frequency depending on the difference between the frequency of the master oscillator and the selected marker frequency. A

Moreover the beat frequency of the master mixerrXl will be either increasing or: decreasing depending fon whether the line tuning motor is moving the master os-Q Vthat the tube is conducting. The tube-V30 and the, associated circuit as shown Yhave the characteristic that when the tube is conducting the cathode beam oscillates back and forth between the two anodes so as to energize first one coil and then the other of the relay S20. As the motor direction control switch S14 (Fig. 2C) is controlled by the relay S20 through the leads 38 and 40 and the contacts S2la ofthe relay S21, the oscillation of the tube V30 causes the motor direction control switch to be oscillated first in one direction and then in the other thereby reversing the direction of the fineftuning motor. When the beat frequency is decreasing, the outputvvoltage of the amplifier A5 decreases thereby discharging the condenser C9 andy causing the right hand plate to become negative. This cuts off the electron beam of the tube V30 and leaves the polarized relay S20 in whichever position it is. As the decreasing beat frequency means that the fine tuning motor is tuning the master oscillator in a direction toward the selected marker frequency, the operation of the tube V30 as just described leaves the fine tuning motor direction control switch in the correct position for the tuning toward the desired frequency to continue. When the selected frequency is reached, the relay S13 under control of the anti-hunt circuit (Fig. 2E) turns off the fine-tuning motor as described above. The master oscillator is thereby tuned to the frequency Vthat has been selected and set up in the control box.

As the` tine tuning slugs 21, 21a, 2lb, etc. are all connected with and operated bythe line tuning motor, the tuning of the master oscillator O1 also results in the tuning of associated units such asV the buffer amplifier, final amplier etc.V If vthe frequency of the master oscillator drifts from the selected frequency, for example because of ternperature variation, the fine tuning system automatically restores it to the proper frequency. As Vthe operating characteristicsof the associated units such as the buffer amplifier, etc.V are preferably the same as those of the master oscillator, the operation of the tine tuning system to correct drift of theV master oscillator simultaneously corrects drift. of Vthe associated units.

While a particular embodiment of the invention has been shown and described it will be understood that the circuit may be modified by those skilled in the art without departing from the spirit of the invention. Thus, for example, other mixers, oscillators, amplifiers, filters etc. may be substituted for the particular units illustrated in the drawings.v It will further be understood by those skilled in the art that certain novel components and combinations of components illustrated and described may be used in other circuitry. Y l

What we claim and desire. to secure by vLetters Patent is:

l. Apparatus for generating a plurality of stable frequencies spaced by predetermined steps overa predeter; mined range, comprising a master oscillator the frequency` of which is variable over the full width of said range and g means for tuning said oscillator to a selected Vfrequency.

in said range comprising a marker oscillator generating Y a `series of marker frequencies spaced apart a multiple ofsaid steps, a master mixer having. inputs connected re-A spectively to said master oscillator and said marker oscillator, a controlv circuit comprising elements that areset-, table Vtoselect a desired frequency, a coarse tuning'motor for said 'master oscillator, a coarse tuning motor circuit controlled ;by said control Vcircuit and controlling :the operation of said .coarse tuning motor to tune said master oscillator to a frequency within a predetermined distance ofthe marker frequency nearest the selected frequency, a `reversible fineV tuning. motorv for said master oscillator and a fine tuningmotor circuit comprising a harmonicsgenerator, means connected to and controlled by said. controlcircuit for selecting a harmonic that algebraically addedto said nearest marker frequency will equal the selected frequency, a direction control circuit connected with the output of saidrmaster mixer and said harmonic selectorY and`v controllingI theY direction of operationyof.

said fine tuning .motrn'L to varyr the frequency of the'master.

oscillator in a direction toward the selected frequency and an anti-hunt circuit, connected with the output of said master mixer and said harmonic selector, for stopping said tne tuning motor when the frequency of the master oscillator equals the algebraic sum of the said nearest marker frequency and the selected harmonic and thus equals the selected frequency.

v 2. Apparatus for generating a plurality of stable frequencies spaced by predetermined steps over a predetermined range, comprising a master oscillator the frequency of which is variable over the full width of said range and means for tuning said oscillator to a selected frequency in said range comprising a marker oscillator generating a series of marker frequencies spaced apart a multiple of said steps, a master mixer having inputs connected respectively to said master oscillator and said marker oscillator, a master control circuit comprising elements that are settable to select a desired frequency, a coarse tuning motor for said master oscillator, a coarse tuning motor circuit controlled by said master control circuit and controlling the operation of said coarse tuning motor to tune said master oscillator to a frequency within a predetermined distance of the marker frequency nearest the selected frequency and on the same side of said marker frequency, a reversible tine tuning motor for said master oscillator and a fine tuning motor circuit comprising a harmonics generator, means connected to and controlled by said master control circuit for selecting a harmonic that algebraically added to said nearest marker frequency will equal the selected frequency, a sweep oscillator having a frequency range at least equal to half the distance between said marker frequencies, a motor direction control circuit connected with the outputs of said master mixer, harmonic selector and sweep oscillator and controlling the direction of operation of said fine tuning motor to vary the frequency of the master oscillator in a direction toward the selected frequency and an antihunt circuit connected to the outputs of said master mixer and harmonic selector for stopping said ne tuning motor when the frequency of the master oscillator equals an algebraic sum of said nearest marker frequency and the selected harmonic and thus equals the selected frequency.

3. Apparatus for generating a plurality of stable frequencies spaced by predetermined steps over a predetern mined range, comprising a master oscillator the frequency of which is variable over the full extent of said range and means for tuning said oscillator to a selected frequency in said range comprising a marker oscillator generating a series of marker frequencies spaced apart a multiple of said steps, a master mixer having inputs connected respectively to said master oscillator and marker oscillator, a master control circuit comprising elements that are settable to select a desired frequency, a coarse tuning motor for said master oscillator, a coarse tuning motor circuit controlled by said master control circuit and controlling the operation of said coarse tuning motor to tune said master oscillator to a frequency within a predeterw mined distance of the marker frequency nearest the selected frequency, a reversible iine tuning motor for said master oscillator and a fine tuning motor circuit cornprising a harmonics generator, means connected to and controlled by said master control circuit for selecting a harmonic that algebraically added to said nearest marker frequency will equal the selected frequency, a sweep oscillator having a range equal to at least half the distance between successive ones of said marker frequencies, a disabling circuit connected to the outputs of said harmonic selector and said sweep oscillator and actuated when the frequency of the sweep oscillator equals that of the harmonic selector, a motor direction control circuit connected to the outputs of said sweep oscillator, master mixer and disabling circuit and operable to reverse the direction of the ne tuning motor when the frequency of the sweep oscillator equals that of the output of the master mixer before that of the harmonic selector and an anti-hunt circuit connected to the outputs of said master mixer and said harmonic selector and operable to stop said tine tuning motor when the frequency of the master oscillator equals the algebraic sum of said nearest marker frequency and the selected harmonic and thus equal the selected frequency.

4. Apparatus for generating a plurality of stable frequencies spaced apart by predetermined steps over a pre determined range, comprising a master oscillator the frequency of which is variable over the full extent of said range and means for tuning said oscillator to a selected frequency in said range comprising a marker oscillator for generating a series of marker frequencies spaced apart a multiple of said steps,A a master mixer having inputs connected respectively to said master oscillator and marker oscillator, a reversible tuning motor for said master oscillator, coarse tuning means for tuning said master oscillator to a `frequency within a predetermined distance of the marker frequency nearest the selected frequency, a harmonics generator, means for selecting from the harmonics produced by said generator a harmonic that algebraically added to said nearest marker frequency will equal the selected frequency, a sweep oscillator having a range equal to at least half the distance between successive ones of said marker frequencies, a disabling circuit connected to the outputs of said harmonic selector and said sweep oscillator and actuated when the frequency of the sweep oscillator equals that of the harmonic selector, a motor direction control circuit connected to the outputs of said sweep oscillator, master mixer and disabling circuit and operable to reverse the direction of the tuning motor when the frequency of the sweep oscillator equals that of the output of the master mixer before that of the harmonic selector andan anti-hunt circuit connected to the outputs of said master mixer and said harmonic selector and operable to stop said tuning motor when the frequency of the master oscillator equals the algebraic sum of said nearest marker frequency and the selected harmonic and thus equals the selected frequency.

5. Apparatus for generating a plurality of stable frequencies spaced by predetermined steps over a predetermined range, comprising a master oscillator the frequency of which is variable over the full extent of said range and means for tuning said oscillator to a selected frequency in said range comprising a marker oscillator generating a series of marker frequencies spaced apart a multiple of said steps, means for selecting a marker frequency near said selected frequency, a master mixer having inputs connected respectively to said master oscillator and marker oscillator, a master control circuit coniprising elements that are settable to select a desired frequency, a reversible tuning motor connected to said master oscillator and a tuning motor circuit comprising a harmonics generator, means connected to and con trolled by said master control circuit for selecting a harmonic that algebraically added to said selected marker frequency will equal the selected frequency, a sweep oscillator having a range equal to at least half the distance between sucessive ones of said marker frequencies, a disabling circuit connected to the outputs of said harmonic selector and said sweep oscillator and actuated when the sweep oscillator frequency equals that of the harmonic selector, a motor direction control circuit connected to the outputs of said sweep oscillator, master mixer and disabling circuit and operable to reverse the direction of the tuning motor when the frequency of the sweep oscillator equals that of the output of the master mixer before that of the harmonic selector and an antihunt circuit connected to the outputs of said master mixer and said harmonic selector and operable to stop said tuning motor when the frequency of the master oscillator :15 Y equals Vthe algebraic sum of siad nearest marker frequency and the selected harmonic and thus equals the selected frequency. l

6. Apparatus for generating a multiplicity of stable frequencies spaced apart by predetermined steps over a .predetermined range, comprising a master oscillator the frequency of which is variable over the full extent of said range and means for tuning said oscillator to a s elected frequency in said range comprising a marker osclllator generating a series of marker frequencies spaced apart a multiple of said steps, a master mixer having 1nputs connected respectively to said master oscillator and marker oscillator, a master control circuit comprising elements for setting up selected impedance values, each impedance value corresponding to one ofthe frequencies in said range, a coarse tuning motor for said master oscillator, a coarse tuning motor circuit comprising a variable impedance connected with said motor and means including an impedance bridge for controlling the operation of said coarse tuning motor and to stop said motor when said variable impedance matches the impedance set up by said master control circuit, said master oscillator being thereby tuned to a frequency within a predetermined distance of the marker frequency nearest the selected frequency, a reversible ne tuning motor for said master oscillator and a fine tuning motor Vcircuit comprising a harmonics generator, means connected to and controlled by said master control circuit for selecting a harmonic that algebraically added to said nearest marker frequency will equal the selected frequency, a direction control circuit connected to the outputs of said master mixer and said harmonic selector and controlling the direction of operation of said tine tuning motor to vary the frequency of the master oscillator in a direction toward the selected frequency and an anti-hunt circuit connected to the outputs of said master mixer and harmonic selector for stopping said ne tuning motor when the frequency of the master oscillator equals the algebraic sum of said nearest marker frequency and the selected harmonic and thus equals the selected frequency.

7. Apparatus for generating a multiplicity of stable frequencies spaced apart by predetermined steps over a predetermined range, comprising a master oscillator the frequency of which is variable over the full extent of said range and means for tuning said oscillator to a selected frequency in said range, a marker oscillator generating a series of marker frequencies spaced apart a multiple of said steps, a master mixer having inputs connected respectively to said master oscillator and marker oscillator, a master control circuit comprising resistance elements for setting up selected resistance values, each resistance value corresponding to one of the frequencies in said range, a coarse tuning motor circuit comprising a doublethrow relay having two opposed operating coils and contacts connected with said motor to reverse the motor upon reversal of said relay, a resistance bridge having four corners and including the resistance set up by said master control circuit and a variable resistance driven by said motor, two triodes having cathodes connected together, platesconnected respectively to the operating coils of said relay, and grids connected across one pair of opposite'rcorners of said bridge, a floating power supply connected vacross the other pair of opposite corners of said bridge, said coarse tuning motor circuit being operable to control the operation of said motor and to stop said motor when said variable resistance matches the resistance set up by said master control circuit, said master oscillator being thereby tuned to a frequency withinV a predetermined distance of the marker frequency nearest the selected frequency, a reversiblene tuning motor for said master oscillator and a iine tuning motor circuit comprising a harmonics generator, means connected to and controlled by said master control circuit for selecting a harmonic that algebraically added to said nearest marker frequency Awill equal the selected frequencyl and means connected to the outputs of -said master mixer and said harmonic selecting means and controlling the direction of operation of said tine tuning motor to vary the frequency of the master oscillator in a direction toward the selected frequency and to stop said fine tuning motor when the frequency of the master oscillator equals the algebraic sum of said nearest marker frequency and the selected harmonic and thus equals the selected frequency. V8. In apparatus for generating a multiplicity of stable frequencies spaced apartY by predetermined steps over a predetermined range, a master oscillator the frequency of which is Variable over the full extent of said range and means for tuning said oscillator to a selected frequency in said range comprising a master control circuit comprising elements for setting up a selected resistance, each resistance value corresponding to one of the frequencies in said range, a reversible tuning motor for said master oscillator, a tuning motor circuit comprising a polar double-throw relay having two opposed operating coils and contacts connected with said motor to reverse the motor upon reversal of said relay, a resistance bridge having four corners and including the resistance set up by said master control circuit and a variable resistance driven by said motor, two triodes having cathodes connected together, plates'connected respectively to the operating coils of said relay and grids connected across one pair of opposite corners of said bridge, a floating power supply'connected across the other pair of opposite corners of said bridge, said tuning motor circuit being operable to control the operation of said motor and to stop saidgmotor when said variable resistance matches the resistance set up by said master controlcircuit, said master oscillator being thereby tuned to the selected frequency.

9. Apparatus according to claim 8, further comprising a second motor ganged with said tuning motor and means for connecting said second motor to operate in parallel with said tuning motor during an initial stage of the tuning operation and to operate as a generator during a subsequent stage and means for feeding the output of said second motor when operating as a generator to said bridge to oppose any potential produced by unbalance of the bridge and thereby act as a damper to prevent overshooting of said tuning motor.

10i. Apparatus for generating a pluralityV of stable frequencies spaced by predetermined steps over a predetermined range, comprising a master oscillator the frequency of which is variable over the full width of said range and means for tuning said oscillator to a selected frequency in said range comprising a marker oscillator generating a series of marker frequencies 4spaced apart a multiple of said steps, a master mixer having inputs connected respectively to said master oscillator and said marker oscillator, a control circuit comprising elements that are settable to select a desired frequency, a coarse tuning motor for said oscillator, a coarse tuning motor circuit controlledv by said control circuit and controlling the operation of said coarse tuning motor to tune said master oscillator to a frequency within a predetermined distance of the marker frequency nearest the selected frequency, a reversible line tuning motor connected to said master oscillator to drive said master oscillator, an auxiliary motor ganged with said Vfine tuning motor, a homing circuit controlling said ne tuning motorand auxiliary motor to bring said master oscillator initially to a selected home position including means for connecting said ne tuning motor and auxiliary motor in parallel during the homing operation, and a ne tuning motor circuit comprising a harmonics generator, means connected to and controlled by said control circuitfor selecting a harmonic that algebraically added to the nearest marker frequency will equal the selected' frequency, a direction control circuit connected to the out-y puts of said master mixer and said harmonic selector and controlling the direction of operation of said fine tuning motor to varythe frequency of said master oscillator in a direction toward the selected frequency and an antihunt circuit connected to the outputs of said master mixer and harmonic selector for stopping said fine tuning motor when the frequency of the master oscillator equals the algebraic sum of said nearest marker frequency and the selected harmonic and thus equals the selected frequency, and means connecting said auxiliary motor to operate as a generator during the fine tuning operation including means providing a load for said generator.

ll. Apparatus for generating a plurality of stable frequencies spaced by predetermined steps over a predetermined range, comprising a master oscillator the frequency of which is variable over the full width of said range and means for tuning said oscillator to a selected frequency in said range comprising a marker oscillator generating a series of marker frequencies spaced apart a multiple of said steps, a master mixer having inputs connected respectively to said master oscillator and said marker oscillator, a reversible tuning motor connected to said master oscillator to drive said master oscillator, an auxiliary motor ganged with said tuning motor, a homing circuit controlling said tuning motor and auxiliary motor to bring said master oscillator initially to a selected home position including means for connecting said tuning motor and auxiliary motor in parallel during the homing operation, coarse tuning means for tuning said master oscillator to a frequency within a predetermined distance of the marker frequency nearest the selected frequency, and a tuning motor circuit comprising a harmonics generator, means connected to and controlled by said control circuit for selecting a harmonic that algebraically added to the nearest marker frequency will equal the selected frequency, a direction control circuit connected to the outputs of said master mixer and said harmonic selector and controlling the direction of operation of said tuning motor to vary the frequency of said master oscillator in a direction toward the selected frequency and means for stopping said tuning motor when the frequency of the master oscillator equals the algebraic sum of said nearest marker frequency and the selected harmonic and thus equals the selected frequency, and means connecting said auxiliary motor to operate as a generator during the tuning operation including means providing a load for said generator.

12. Apparatus for generating a plurality of stable frequencies spaced by predetermined steps over a predetermined range, comprising a plurality of master oscillators the frequency of which is variable over the full width of the selected range, a master mixer associated with each of said oscillators, a tuning motor for each of said oscillators, a single tuning circuit for selectively tuning all of said oscillators and means for selectively connecting the master mixer and tuning motor of a selected master oscillator to said tuning circuit for tuning said selected oscillator, said tuning circuit comprising a marker oscillator generating a series of marker frequencies in said range and spaced apart a multiple of said steps, means connecting the inputs of the selected master mixer with the respective master oscillator and said marker oscillator, a harmonics generator producing harmonics spaced apart the same distance as said steps, means for selecting from the harmonics generated by said harmonics generator a harmonic that algebraically added to the nearest marker frequency will equal the selected frequency and a motor control circuit connected with the output of the selected master mixer and said harmonic selector and controlling the operation of the selected tuning motor to tune the master oscillator to a frequency which equals the algebraic sum of said nearest marker frequency and the selected harmonic and thus equals the selected frequency.

13. Apparatus for generating a plurality of stable frequencies spaced by predetermined steps over a predetermined -range, comprising a master oscillator the frequency of which is variable over the full width of said range and means for tuning said oscillator to a selected frequency in said range comprising a marker oscillator generating a series f marker frequencies spaced apart a multiple of said steps, a master mixer having inputs connected respectively to said master oscillator and said marker oscillator, a control circuit comprising elements that are settable to select a desired frequency, a tuning motor for said master oscillator, coarse tuning means connected to and controlled by said control circuit for tuning said master oscillator to a frequency within a predetermined distance of the marker frequency nearest the selected frequency, a harmonics generator, means connected to and controlled by said control circuit for selecting from the harmonics produced by said gen erator a harmonic that algebraically added to said nearest marker frequency will equal the selected frequency, including means determining whether the selected harmonic is added to or subtracted from said marker frequency, and a motor control and reversing circuit connected with the output of said master mixer and said harmonic selector and controlling the operation of said tuning motor to vary the frequency of the master oscillator in a direction toward the selected frequency and to stop said tuning motor when the frequency of the master oscillator equals the algebraic sum of said nearest marker frequency and the selected harmonic and thus equals the selected frequency.

i4. Apparatus for generating a plurality of stable fre quencies spaced by predetermined steps over a predetermined range, comprising a master oscillator the frequency of which is variable over the full extent of said range and means for tuning said` oscillator to a selected frequency in said range comprising a marker oscillator generating a series of marker frequencies spaced apart a multiple of said steps, a master mixer having inputs connected respectively to said master oscillator and marker oscillator, a ltuning motor for said master oscillator, coarse tuning means for tuning said master oscillator to a frequency within a predetermined distance of the marker frequency nearest the selected frequency, a harmonics generator, means for selecting a harmonic that algebraica'lly added to said nearest marker frequency will equal the selected frequency, two circuits for controlling the direction of said motor to tune said master oscillator in a direction toward the selected frequency, one of said direction control means being connected to the output of said harmonic selecting means and operable when the selected frequency is spaced from .a marker frequency and the second of said direction control means being connected to the output of said master mixer and operable when the selected frequency coincides with one of the marker frequencies, means for selecting one or the other of said direction control means and an anti-hunt circuit connected to the outputs of said master mixer and said harmonic selecting means and operable to stop said tuning motor when the frequency of the master oscillator equals the selected frequency.

l5. Apparatus according to claim lll, in which said second motor direction control means comprises an amplifier connected to the output of said master mixer, a rectifier in the output circuit of said amplifier, a sheet beam tube having a cathode, control grid and two anodes, a condenser having one plate connected to the rectified output of said amplifier and the other connected to said control grid, a double-throw polarized relay having operating coils connected respectively to said anodes and connections between said relay and said tuning motor whereby the direction of the motor is controlled by said relay, said sheet beam tube and associated circuit having the characteristic that the beam oscillates bacl: and forth between said anodes when the tube is conducting.

l5. Frequency controlling apparatus comprising a tunable element the frequency of which is to be controlled, a tuning motor for said element, a source of standard 't9 frequencies, a mixer having inputs connected respectively to said tunable element and said standard frequency source, means for selecting a frequency from said standard frequency source to which said tunable element is to be tuned, means for controlling the direction of said tuning motor to tune said element in a direction toward the selected frequency, said direction control means comprising an amplier connected to the output of said master mixer, a rectifier in the output circuit of said amplier, a sheet beam tube having a cathode, control grid and two anodes, a condenser having one plate connected to the rectied output of said arnplier and the other connected to said grid control, `a double-throw polarized relay having operating coils connected respectively to said anodes and connections between said relay and said tuning motor References Cited in the le of this patent UNlTED STATES PATENTS 2,281,46l Smith Apr. 28, 1942 2,483,070 Spindler Sept. 27, 1949 2,523,106 Fairbairn et al Sept. 19, 1950 2,581,594 MacSorley Ian. 8, 1952 

